An aromatic vinyl-conjugated diene-aromatic vinyl block copolymer (i.e., thermoplastic elastomer) exhibits excellent elasticity and flexibility, and is used as a material for producing an elastic film that is used when producing a sanitary article (e.g., disposable diaper and sanitary item), for example.
A sanitary article (e.g., disposable diaper and sanitary item) is required to follow the motion of the wearer, and fit the wearer, and an elastic film is used to form various parts of the sanitary article. For example, an elastic film is provided around the thigh opening, around the waist opening, on the side part, and the like when producing a pull-up diaper (i.e., disposable diaper). Since it is necessary to prevent the displacement of the sanitary article even when the wearer makes a large motion, or wears the sanitary article for a long time, an elastic film used for such an application is required to have a high modulus of elasticity and exhibit low tension set.
Patent Literature 1 and 2 disclose a block copolymer composition that includes an asymmetric aromatic vinyl-conjugated diene-aromatic vinyl block copolymer in which the aromatic vinyl polymer blocks differ in weight average molecular weight (weight average molecular weight range), and an aromatic vinyl-conjugated diene-aromatic vinyl block copolymer having a specific configuration that differs from the configuration of the asymmetric aromatic vinyl-conjugated diene-aromatic vinyl block copolymer, as a material formed of an aromatic vinyl-conjugated diene-aromatic vinyl block copolymer that can meet the above demand. The block copolymer compositions disclosed in Patent Literature 1 and 2 have a high modulus of elasticity, exhibit low tension set, and exhibit mechanical properties suitable for a material for forming an elastic film used for a sanitary article.
However, the formation of a hole may occur when a film is formed by melt-forming the block copolymer composition specifically disclosed in Patent Literature 1 or 2. When a hole is formed in an elastic film used for a sanitary article, the mechanical properties of the film are impaired, and breakage of the film may occur, for example. Therefore, it is strongly desired to prevent the formation of a hole.